This invention relates generally to magnetic encoders and more particularly to means for producing stable and accurate multiple phase shifted signals in such encoders by placing magnetoresistive (MR) elements in the encoders at A/n intervals, wherein n.gtoreq.8, of the recording pitch, A, of a magnetic recording medium, such as a drum, tape, disk or the like.
Shown in FIGS. 2A through 2C is a magnetic encoder of the prior art and is also disclosed in U.S. Pat. No. 4,594,548. In this encoder, magnetoresistive (MR) elements, R.sub.x, form sensor 2 and are placed at A/4 pitch intervals, relative to signal recording wavelength or recording pitch, A, of magnetic recording medium 1. The encoder circuit 10 for sensor 2 is disclosed in FIGS. 2A and 2B and comprises a pair of bridge circuits 10A and 10B wherein MR elements R.sub.1, R.sub.3, R.sub.5 and R.sub.7 are connected to form bridge circuit 10A and MR elements R.sub.2, R.sub.4, R.sub.6 and R.sub.8 are connected to form bridge circuit 10B. The outputs of bridge circuits 10 are connected as inputs to multiplier circuit 14, shown in FIG. 3, to amplify the sensor analog signals via amplifying stages 11 to form amplified phase shifted analog signals, I and J, shown in FIG. 4, which are then converted into phase shifted, pulse output signals, G and H, shown in FIG. 4, via operational amplifiers or comparators 3 and pull down transistors 12 of circuit 14.
When high resolution output signals are required relative to magnetic recording medium 1, recording pitch, A, of recording medium 1 is reduced for recording magnetic signals. In such a case, it is the practice to apply different comparative reference voltages by varying the input reference voltages at 13 to comparators 3 and the desired phase shifted signals of sensors 2 are derived via the outputs of comparators 3 and pull down transistors 12. However, as recognized in U.S. Pat. No. 4,594,548, when recording pitch, A, of recording medium 1 is reduced or made shorter to improve recording resolution, the magnetic field strength to MR sensor 2 is reduced. Unless gap 15 between recording medium 1 and MR sensor 2 is also correspondingly decreased, the output signals from the MR sensor 2 and its bridge circuits 10 would be too small and weak for amplification and processing via multiplier circuit 14.
Furthermore, if gap 15 is constructed or adjusted to be too small to accommodate changes toward providing for higher resolution, assembly and adjustment of the encoder is made difficult.
Also, in the case where changes are made to applied input reference voltages to comparators 3 in order to phase shift the output signals of MR sensor 2, the output signals of MR sensor 2 are easily affected by changes in environmental and ambient conditions since these signals from their initial creation, by their very nature, are weak. Furthermore, each comparative reference voltage set at reference voltage 13 has to be accurately adjusted and correct adjustment is not easily maintained on a continuous basis. Also, the duty ratio of output signals, G and H, fluctuate according to changes in circuit and ambient conditions, such as circuit voltages and temperatures and humidity. In the worst case, multiple phase shifted signals, G and H, are not accurately produced due to such changing conditions.
It is an object of this invention is to provide a high resolution magnetic encoder capable of providing stable and accurate multiple phase shifted signals in spite of large changes in recording pitch, A, of a magnetic recording medium, particularly changes in reduction of recording pitch, A, to obtain high resolution recording of data signals on the magnetic recording medium.
It is another object of this invention to improve the accuracy of detection of a magnetic encoder while reducing the recording pitch, A, without the need to make any changes to the sensor/medium gap by increasing the number of MR elements, corresponding MR element bridge circuits and the resultant plurality of derived phase shifted outputs to produce a set or pair of desired output phase shift signals.